1. Field
Exemplary embodiments of the present invention relate to a data transmission circuit.
2. Description of the Related Art
In general, semiconductor memory devices, such as dynamic random access memory (DRAM), include data transmission circuits for outputting data. Furthermore high-speed data transmission circuits for outputting data at giga-bit per second (Gbps) speeds adopt methods of data emphasis to reduce inter symbol integrity (ISI) jitter. A data emphasis method is described below.
FIG. 1 is a schematic diagram illustrating gains of a data transmission circuit 110, a transmission line 120, and a data output circuit 130 without data emphasis (the case “(a)”) and with data emphasis (the case “(b)”).
As illustrated in FIG. 1, the data transmission line 120 has a low pass filter characteristic Tx_Line due to parasitic resistance R and parasitic capacitance C. Accordingly, without data emphasis, input data DATA_IN transmitted by the data transmission circuit 110 is distorted. The data output circuit 130 outputs output data DATA_OUT in which the gain of a high frequency part has a low level.
According to the data emphasis, the transmission circuit 110 emphasizes a high frequency component of the input data DATA_IN by taking signal distortion into consideration and transmitting the input data DATA_IN. Case “(b)” shows that the transmission circuit 110 emphasizes the high frequency component of the input data DATA_IN and outputs the output data DATA_OUT in which the gain of the high frequency part has the original level. That is, although the transmission line 120 has a low pass filter characteristic Tx_Line, the data output circuit 130 may receive signals having the same gain
FIG. 2 is a waveform illustrating data according to data emphasis in a conventional data transmission circuit.
As illustrated in FIG. 2, according to the data emphasis, the data DATA is emphasized when the level of the data shifts. When the level of the data DATA shifts from a logic “low” to a logic “high”, the “high” level is emphasized for a predetermined time. When the level of the data DATA shifts from a logic “high” to a logic “low”, the “low” level is also emphasized. The high frequency component of the data DATA is concentrated at the shifting point of the data DATA level, and thus the level of data DATA is emphasized on the high frequency component when the level of the data DATA shifts.
As illustrated in FIG. 2, when the data DATA shifts, the level of the data DATA is emphasized for a single unit interval (UI) of the data DATA.